I. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to improving a communication receiver's immunity to interference.
II. Description of the Related Art
There are presently multiple types of cellular radiotelephone systems operating. These systems include the advanced mobile phone system (AMPS) and the two digital cellular systems: time division multiple access (TDMA) and code division multiple access (CDMA). The digital cellular systems are being implemented to handle capacity problems that AMPS is experiencing.
All the cellular radiotelephone systems operate by having multiple antennas covering a geographic area. The antennas radiate into an area referred to in the art as a cell. The AMPS cells are separate and distinct from the CDMA cells so that the cells of each system overlap. This makes it likely that the antenna for one system's cell may be located in a cell of another system. Likewise, within a particular system (AMPS, CDMA, and TDMA), there are two service providers within a given area. These providers often choose to place cells in different geographical locations from their competitor, hence there are situations where a radiotelephone on system `A` might be far away from the nearest system `A` cell while close to a system `B` cell. This situation means that the desired receive signal will be weak in the presence of strong multi-tone interference.
This intermixing of system antennas can cause problems for a mobile radiotelephone that is registered in one system, such as the CDMA system, and travels near another system's antenna, such as an AMPS antenna. In this case, the signals from the AMPS antenna can interfere with the CDMA signals being received by the radiotelephone due to the proximity of the radiotelephone with the AMPS cell or the higher power of the AMPS forward link signal.
The multi-tone interference encountered by the radiotelephone from the AMPS signals creates distortion products or spurs. If these spurs fall in the CDMA band used by the radiotelephone, they can degrade receiver and demodulator performance.
It is frequently the case in an AMPS system for the carriers (A and B bands) to `jam` the competitor system unintentionally. The goal of the cellular carrier is to provide a high signal to noise ratio for all the users of their system by placing cells close to the ground, or near their users, and radiating the FCC power limit for each AMPS channel. Unfortunately, this technique provides for better signal quality for the carrier's system at the expense of interfering with the competitor's system.
Intermodulation distortion, such as that caused by the above situations, is defined in terms of the peak spurious level generated by two or more tones injected into a receiver. Most frequently, the third-order distortion level is defined for a receiver in terms of a third-order input intercept point or IIP3. IIP3 is defined as the input power (in the form of two tones) required to create third order distortion products equal to the input two tone power. As shown in FIG. 13, IIP3 can only be linearly extrapolated when a non-linear element, such as an amplifier, is below saturation.
As shown in FIG. 14, third-order distortion products occur when two tones are injected in a receiver. Tone #1 is at frequency f.sub.1 at power level P.sub.1 in dBm. Tone #2 is at frequency f.sub.2 at power level P.sub.2 in dBm. Typically P.sub.2 is set to equal P.sub.1. Third-order distortion products will be created at frequencies 2.times.f.sub.1 -f.sub.2 and 2.times.f.sub.2 -f.sub.1 at power levels P.sub.12 and P.sub.21 respectively. If P.sub.2 is set to equal P.sub.1, then spurious products should be equal, or P.sub.12 and P.sub.21 should be equal. Signal f.sub.c is injected at power level P.sub.c to show that the added distortion is equal to a low level signal in this case. If there is a filter that filters out f.sub.1, f.sub.2 and f.sub.21 after the distortion is created, the power at f.sub.12 will still interfere with the signal power at f.sub.c. In example FIG. 14, for a CDMA application, the goal is that the intermod P.sub.12 should be equal to the signal power of -105 dBm for a total two tone power of -43 dBm, so the IIP3 must be &gt;-9 dBm.
As is well known in the art, IIP3 for a single non-linear element is defined as the following: ##EQU1## EQU If P.sub.1 =P.sub.2, then P.sub.in =P.sub.1 +3 dB or P.sub.2 +3 dB (dBm)
and EQU IM3=P.sub.1 -P.sub.12 =P.sub.2 -P.sub.21 =P.sub.2 -P.sub.12 =P.sub.1 -P.sub.21 (dB)
For cascaded IIP3, where more non-linear elements are used, the equation is as follows: EQU IIP3=-10*log10 10.sup.(Gain-element IIP3)/1O +10.sup.(-IIP3 of previous stages)/10 ! PA1 NFe equals the noise figure of the element, PA1 NFi equals the cascaded noise figure up to the element, and PA1 Gain equals the running gain up to the element. PA1 where EQU -61 dBm/Hz is the noise bandwidth for a CDMA channel EQU -45 dBm/Hz is the noise bandwidth for a FM channel
where: Gain=gain to element input.
Therefore, one way to improve the cascaded IIP3 of a receiver is to lower the gain before the first non-linear element. In this case, the LNA and mixer limit IIP3. However, another quantity needs to be defined that sets the sensitivity or lowest receive signal level without interference. This quantity is referred to in the art as the noise figure (NF). If the gain of the receiver is reduced to improve IIP3 (and interference immunity), the NF (and sensitivity to small desired signals) is degraded.
The Element NF is defined as the following: ##EQU2## where: ##EQU3## is the input signal to noise ratio in dB, and ##EQU4## is the output signal to noise ratio in dB.
For elements in cascade in a receiver, the equation is as follows: ##EQU5##
where:
The `best` cascaded NF can be achieved if the gain up to the element is maximized, this equation is in contradiction to the requirement for the `best` cascaded IIP3. For a given element by element and receiver NF and IIP3, there are a limited set of gain values for each element that meet all of the requirements.
Typically, a receiver is designed with NF and IIP3 as predefined constants, as both of these quantities set the receiver's dynamic range of operation with and without interference. The gain, NF, & IIP3 of each device are optimized based on size, cost, thermal, quiescent and active element current consumption. In the case of a dual-mode CDMA/FM portable cellular receiver, the CDMA standard requires a 9 dB NF at minimum signal. In other words, for CDMA mode, the sensitivity requirement is a 0 dB S/N ratio at -104 dBm. For FM mode, the requirement is a 4 dB S/N ratio at -116 dBm. In both cases, the requirements can be translated to a NF as follows: ##EQU6## where S is the minimum signal power, ##EQU7## is the minimum signal to noise ratio, N.sub.therm is the thermal noise floor (-174 dBm/Hz @ 290.degree. K), and Signal BW (dB/Hz) is the bandwidth of the signal.
Therefore, EQU CDMA NF=-104 dBm-0 dB-(-174 dBm/Hz)-61 dB/Hz=9 dB, EQU FM NF=-116 dBm-4 dB-(-174 dBm/Hz)-45 dB/Hz=9 dB,
However, the receiver's NF is only required when the signal is near the minimum level and the IIP3 is only required in the presence of interference or strong CDMA signals.
There are only two ways to provide coverage in the areas where the carrier is creating strong interference. One solution is to employ the same technique; i.e., co-locate their cells along with the competition's. Another solution is to improve the immunity of a receiver to interference. One way to improve the immunity is to increase the receiver current. This is not a practical solution, however, for a portable radio that relies on battery power. Increasing the current would drain the battery more rapidly, thereby decreasing the talk and standby time of the radiotelephone. There is a resulting need to minimize multi-tone interference in a radiotelephone without impacting the current consumption.